Pluto Facts - Interesting Facts about Dwarf Planet Pluto Pluto Facts Pluto – Discovered in 1930, Pluto is the second closest dwarf planet to the Sun and was at one point classified as the ninth planet. Pluto is the largest dwarf planet but only the second most massive, with Eris being the most massive. 1.31 × 10^22 kg (0.17 Moons) Orbit Distance: Pluto size compared to the Moon and Earth Pluto’s Distance From Sun Facts about Pluto Pluto is named after the Greek god of the underworld. This is a later name for the more well known Hades and was proposed by Venetia Burney an eleven year old schoolgirl from Oxford, England. Pluto was reclassified from a planet to a dwarf planet in 2006.

This is when the IAU formalised the definition of a planet as “A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.” Pluto was discovered on February 18th, 1930 by the Lowell Observatory. For the 76 years between Pluto being discovered and the time it was reclassified as a dwarf planet it completed under a third of its orbit around the Sun. Pluto has five known moons. The moons are Charon (discovered in 1978,), Hydra and Nix (both discovered in 2005), Kerberos originally P4 (discovered 2011) and Styx originally P5 (discovered 2012) official designations S/2011 (134340) 1 and S/2012 (134340) 1. Pluto is the largest dwarf planet. At one point it was thought this could be Eris.

Currently the most accurate measurements give Eris an average diameter of 2,326km with a margin of error of 12km, while Pluto’s diameter is 2,372km with a 2km margin of error. Pluto is one third water. This is in the form of water ice which is more than 3 times as much water as in all the Earth’s oceans, the remaining two thirds are rock. Pluto’s surface is covered with ices, and has several mountain ranges, light and dark regions, and a scattering of craters. Pluto is smaller than a number of moons. These are Ganymede, Titan, Callisto, Io, Europa, Triton, and the Earth’s moon . Pluto has 66% of the diameter of the Earth’s moon and 18% of its mass. While it is now confirmed that Pluto is the largest dwarf planet for around 10 years it was thought that this was Eris. Pluto has a eccentric and inclined orbit. This takes it between 4.4 and 7.3 billion km from the Sun meaning Pluto is periodically closer to the Sun than Neptune. Pluto has been visited by one spacecraft.

The New Horizons spacecraft, which was launched in 2006, flew by Pluto on the 14th of July 2015 and took a series of images and other measurements. New Horizons is now on its way to the Kuiper Belt to explore even more distant objects. Pluto’s location was predicted by Percival Lowell in 1915. The prediction came from deviations he initially observed in 1905 in the orbits of Uranus and Neptune. Pluto sometimes has an atmosphere. When Pluto elliptical orbit takes it closer to the Sun, its surface ice thaws and forms a thin atmosphere primarily of nitrogen which slowly escapes the planet. It also has a methane haze that overs about 161 kilometres above the surface. The methane is dissociated by sunlight into hydrocarbons that fall to the surface and coat the ice with a dark covering. When Pluto travels away from the Sun the atmosphere then freezes back to its solid state. When Was the Dwarf Planet Pluto Discovered? By Jennifer Rosenberg Updated March 03, 2016. On February 18, 1930, Clyde W.

Tombaugh, an assistant at the Lowell Observatory in Flagstaff, Arizona, discovered Pluto. For over seven decades, Pluto was considered the ninth planet of our solar system. The Discovery It was American astronomer Percival Lowell who first thought there might be another planet somewhere near Neptune and Uranus. Lowell had noticed that the gravitational pull of something large was affecting the orbits of those two planets. However, despite looking for what he called "Planet X" from 1905 until his death in 1916, Lowell never found it. Thirteen years later, the Lowell Observatory (founded in 1894 by Percival Lowell) decided to recommence Lowell's search for Planet X. They had a more powerful, 13-inch telescope built for this sole purpose. The Observatory then hired 23-year-old Clyde W. Tombaugh to use Lowell's predictions and the new telescope to search the skies for a new planet. It took a year of detailed, painstaking work, but Tombaugh did find Planet X.

continue reading below our video 4 Tips for Improving Test Performance The discovery occurred on February 18, 1930 while Tombaugh was carefully examining a set of photographic plates created by the telescope. Despite Planet X being discovered on February 18, 1930, the Lowell Observatory was not quite ready to announce this huge discovery until more research could be done. After a few weeks, it was confirmed that Tombaugh's discovery was indeed a new planet. On what would have been Percival Lowell's 75th birthday, March 13, 1930, the Observatory publicly announced to the world that a new planet had been discovered. Pluto the Planet Once discovered, Planet X needed a name. Everyone had an opinion. However, the name Pluto was chosen on March 24, 1930 after 11-year-old Venetia Burney in Oxford, England suggested the name "Pluto." The name denotes both the assumed unfavorable surface conditions (as Pluto was the Roman god of the underworld) and also honors Percival Lowell, as Lowell's initials make up the first two letters of the planet's name.

At the time of its discovery, Pluto was considered to be the ninth planet in the solar system. Pluto was also the smallest planet, being less than half the size of Mercury and two-thirds the size of Earth's moon. Usually, Pluto is the planet farthest from the sun. This great distance from the sun makes Pluto very inhospitable; it's surface is expected to be made up of mostly ice and rock and it takes Pluto 248 years just to make one orbit around the sun. Pluto Loses Its Planet Status As the decades passed and astronomers learned more about Pluto, many questioned whether Pluto could really be considered a full-fledged planet. Pluto's status was questioned in part because it was by far the smallest of the planets. Plus, Pluto's moon (Charon, named after Charon of the underworld , discovered in 1978) is incredibly large in comparison. Pluto's eccentric orbit also concerned astronomers; Pluto was the only planet whose orbit actually crossed that of another planet (sometimes Pluto crosses Neptune's orbit).

When bigger and better telescopes began to discover other large bodies beyond Neptune in the 1990s, and especially when another large body was discovered in 2003 that rivaled the size of Pluto, Pluto's planet status became seriously questioned . In 2006, the International Astronomical Union (IAU) officially created a definition of what makes a planet; Pluto did not meet all the criteria. Pluto was then downgraded from a "planet" to a "dwarf planet." Pluto: Facts & Information About the Dwarf Planet Pluto A wide view of Pluto taken by the New Horizons space probe in July 2015. Credit: NASA/APL/SwRI Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar system. It is also one of the largest known members of the Kuiper Belt , a shadowy disklike zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets.

In 2006, Pluto was reclassified as a dwarf planet , a change widely thought of as a demotion. The question of Pluto's planet status has attracted controversy and stirred debate in the scientific community , and among the general public , since then. American astronomer Percival Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at these two planets from beyond. Lowell predicted the mystery planet's location in 1915, but died without finding it. Pluto was finally discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers. Pluto got its name from an 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto.

Physical characteristics Since Pluto is so far from Earth, little was known about the dwarf planet's size or surface conditions until 2015, when NASA's New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide as Earth's moon. Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features , including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice. [ Photos of Pluto and Its Moons ] Pluto's surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet's surface.

Another distinct feature on Pluto's surface is a large heart-shaped region known unofficially as Tombaugh Reggio (after Clyde Tombaugh). The left side of the reggio (an area that takes on the shape of an ice cream cone) is covered in carbon monoxide ice. Other variations in the composition of surface materials have been identified within the "heart" of Pluto. In the center left of Tombaugh Reggio is a very smooth region unofficially known by the New Horizons team as "Sputnik Planum," after Earth's first artificial satellite, Sputnik. This region of Pluto's surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It's possible that this region is still being shaped and changed by geologic processes . These icy plains also display dark streaks that are a few miles long, and aligned in the same direction. It's possible the lines are created by harsh winds blowing across the dwarf planet's surface. NASA's Hubble Space Telescope has also revealed evidence that Pluto's crust could contain complex organic molecules .

Pluto's surface is one of the coldest places in the solar system, at roughly minus 375 degrees Fahrenheit (minus 225 degrees Celsius). When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes. Orbital characteristics Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. Since the dwarf planet's orbit is so eccentric, or far from circular, Pluto's distance from the sun can vary considerably. The dwarf planet actually gets closer to the sun than Neptune is for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world. As a result of that orbit, after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet). When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane.

Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds. A close-up view of the surface of Pluto, taken by the New Horizons space probe in July, 2015, revealed the presence of icy mountains on the dwarf planet's surface. Credit: NASA-JHUAPL-SwRI Composition & structure Atmospheric composition : Methane, nitrogen. Observations by New Horizons show that Pluto's atmosphere extends as far as 1,000 miles (1,600 km) above the surface of the dwarf planet. Magnetic field: It remains unknown whether Pluto has a magnetic field, but the dwarf planet's small size and slow rotation suggest it has little to no such field. Chemical composition: Pluto probably consists of a mixture of 70 percent rock and 30 percent water ice.

Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface. Orbit & rotation Pluto's rotation is retrograde compared to the solar systems' other worlds; it spins backward, from east to west.

Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth Pluto's moons Pluto has five moons: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closest to Pluto and Hydra the most distant. In 1978, astronomers discovered that Pluto had a very large moon nearly half the dwarf planet's own size. This moon was dubbed Charon, after the mythological demon who ferried souls to the underworld in Greek mythology.

Because Charon and Pluto are so similar in size, their orbit is unlike that of most planets and their moons. Both Pluto and Charon orbit a point in space that lies between them, similar to the orbits of binary star systems , For this reason, scientists refer to Pluto and Charon as a double dwarf planet, double planet or binary system. Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking. While Pluto has a reddish tint, Charon appears more grayish. In its early days, the moon may have contained a subsurface ocean , though the satellite probably can't support one today. Compared with most of the solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun.

Observations of Charon by New Horizons have revealed the presence of canyons on the moon's surface. The deepest of those canyons plunge downward for up to 6 miles (9.7 km). A long swatch of cliffs and troughs stretches for 600 miles (970 km) across the middle of the satellite. A section of the moon's surface near one pole is covered in a much darker material than the rest of the planet. Similar to regions of Pluto, much of Charon's surface is free of craters — suggesting the surface is quite young and geologically active . In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission and discovered two other tiny moons of Pluto , now dubbed Nix and Hydra. These satellites are two and three times farther away from Pluto than is Charon. Based on measurements by New Horizons, Nix is estimated to be 26 miles (42 km) long and 22 miles (36 km) wide, while Hydra is estimated at 34 miles (55 km) long and 25 miles (40 km) wide.

It is likely that Hydra's surface is coated primarily in water ice. Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. On July 11, 2012, a fifth moon , Styx, was discovered (with an estimated width of 6 miles or 10 km), further fueling the debate about Pluto's status as a planet. The four newly spotted moons may have formed from the collision that created Charon. Their orbits have been found to be highly chaotic . Research & exploration NASA's New Horizons mission is the first probe to study Pluto, its moons and other worlds within the Kuiper Belt up close. It was launched on January 2006, and successfully made its closest approach to Pluto on July 14, 2015. It will take 16 months to download all the data that the spacecraft collected during its flyby of Pluto and its moons. Mission leaders say they would like to send New Horizons to study another object in the Kuiper belt, but NASA has not yet approved an extended mission.

The New Horizons probe carries some of the ashes of Pluto's discoverer , Clyde Tombaugh. The limited knowledge of the Pluto system created unprecedented dangers for the New Horizons probe. Prior to the mission's launch, scientists knew of the existence of only three moons around Pluto. The discovery of Kerberos and Styx during the spacecraft's journey fueled the idea that more satellites could orbit the dwarf planet, unseen from Earth . Collisions with unseen moons, or even small bits of debris, could have seriously damaged the spacecraft. But the New Horizons design team equipped the space probe with tools to protect it during its journey. Pluto's formation & origins The leading hypothesis for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-size object. Most of the combined matter became Pluto, while the rest spun off to become Charon, this idea suggests. [ Infographic: Pluto: A Dwarf Planet Oddity ] Additional reporting by staff writer Calla Cofield and Nola Taylor Redd, Space.com contributor. Additional resources The International Astronomical Union makes its case for demoting Pluto to a dwarf planet. Track the New Horizons spacecraft as it it continues on its journey into the Kuiper Belt.

Learn more about Pluto at NASA's Dwarf Planets page . Related: Dwarf Planet Pluto Dwarf Planet Pluto Come wander with me, she said, Into regions yet untrod; And read what is still unread In the manuscripts of God. - Longfellow Copyright © 1998-2015 by Rosanna L. Hamilton. All rights reserved. Table of Contents Although Pluto was discovered in 1930, limited information on the distant object delayed a realistic understanding of its characteristics. Pluto is the second largest known dwarf planet and tenth largest orbiting the Sun. From its time of discovery in 1930 to 2006 it was considered to be the ninth planet in the solar system, but because additional objects have been discovered including Eris which is 27% more massive, the IAU reclassified Pluto and the other objects as dwarf planets. The New Horizons spacecraft was launched on January 16, 2006 and will make its closest approach to Pluto on July 14, 2015. This mission will provide an increased amount of information about this peculiar dwarf planet.

The uniqueness of Pluto's orbit, rotational relationship with its satellite, spin axis, and light variations all give it a certain appeal. Pluto is usually farther from the Sun than any of the eight planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit. Pluto crossed Neptune's orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226. As Pluto approaches perihelion it reaches its maximum distance from the ecliptic due to its 17-degree inclination. Thus, it is far above or below the plane of Neptune's orbit. Under these conditions, Pluto and Neptune will not collide and do not approach closer than 18 A.U. to one another. Pluto's rotation period is 6.387 days, the same as its satellite Charon. Although it is common for a satellite to travel in a synchronous orbit with its planet, Pluto rotates synchronously with the orbit of its satellite.

Thus being tidally locked, Pluto and Charon continuously face each other as they travel through space. Unlike most planets, but similar to Uranus , Pluto rotates with its poles almost in its orbital plane. Pluto's rotational axis is tipped 122 degrees. When Pluto was first discovered, its relatively bright south polar region was the view seen from the Earth. Pluto appeared to grow dim as our viewpoint gradually shifted from nearly pole-on in 1954 to nearly equator-on in 1973. Pluto's equator is now the view seen from Earth . During the period from 1985 through 1990, Earth was aligned with the orbit of Charon around Pluto such that an eclipse could be observed every Pluto day. This provided opportunity to collect significant data which led to albedo maps defining surface reflectivity, and to the first accurate determination of the sizes of Pluto and Charon, including all the numbers that could be calculated therefrom. The first eclipses (mutual events) began blocking the north polar region. Later eclipses blocked the equatorial region, and final eclipses blocked Pluto's south polar region. By carefully measuring the brightness over time, it was possible to determine surface features.

It was found that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Pluto's geometric albedo is 0.49 to 0.66, which is much brighter than Charon. Charon's albedo ranges from 0.36 to 0.39. The eclipses lasted as much as four hours and by carefully timing their beginning and ending, measurements for their diameters were taken. The diameters can also be measured directly to within about 1 percent by more recent images provided by the Hubble Space Telescope. These images resolve the objects to clearly show two separate disks. The improved optics allow us to measure Pluto's diameter as 2,274 kilometers (1413 miles) and Charon's diameter as 1,172 kilometers (728 miles), just over half the size of Pluto. Their average separation is 19,640 km (12,200 miles). That's roughly eight Pluto diameters. Average separation and orbital period are used to calculate Pluto and Charon's masses.

Pluto's mass is about 6.4 x 10-9 solar masses. This is close to 7 (was 12 x's) times the mass of Charon and approximately 0.0021 Earth mass, or a fifth of our moon. Pluto's average density lies between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Charon's density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon's numbers derived from HST data are still being challenged by ground based observations. Pluto and Charon's origin remains in the realm of theory. Pluto's icy surface is 98% nitrogen (N2). Methane (CH4) and traces of carbon monoxide (CO) are also present. The solid methane indicates that Pluto is colder than 70 Kelvin.

Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun. The atmospheric pressure deduced for Pluto's surface is 1/100,000 that of Earth's surface pressure. Pluto was officially labeled the ninth planet by the International Astronomical Union in 1930 and named for the Roman god of the underworld. It was the first and only planet to be discovered by an American, Clyde W. Tombaugh. It has since been reclassified as a Dwarf Planet along with Eris and Ceres . The path toward its discovery is credited to Percival Lowell who founded the Lowell Observatory in Flagstaff, Arizona and funded three separate searches for "Planet X." Lowell made numerous unsuccessful calculations to find it, believing it could be detected from the effect it would have on Neptune's orbit. Dr.

Vesto Slipher, the observatory director, hired Clyde Tombaugh for the third search and Clyde took sets of photographs of the plane of the solar system (ecliptic) one to two weeks apart and looked for anything that shifted against the backdrop of stars. This systematic approach was successful and Pluto was discovered by this young (born 4 Feb 1906) 24 year old Kansas lab assistant on February 18, 1930. Pluto is actually too small to be the "Planet X" Percival Lowell had hoped to find. Pluto's was a serendipitous discovery. Pluto Statistics Pluto's Big Heart in Color Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons spacecraft, taken on July 13, 2015, when the spacecraft was 476,000 miles (768,000 kilometers) from the surface. This is the last and most detailed image sent to Earth before the spacecraft's closest approach to Pluto on July 14.

The color image has been combined with lower-resolution color information from the Ralph instrument that was acquired earlier on July 13. This view is dominated by the large, bright feature informally named the "heart"Â which measures approximately 1,000 miles (1,600 kilometers) across. The heart borders darker equatorial terrains, and the mottled terrain to its east (right) is complex. However, even at this resolution, much of the heart's interior appears remarkably featureless -- possibly a sign of ongoing geologic processes. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Pluto: The Ice Plot Thickens The latest spectra from New Horizons Ralph instrument reveal an abundance of methane ice, but with striking differences from place to place across the frozen surface of Pluto. In the north polar cap, methane ice is diluted in a thick, transparent slab of nitrogen ice resulting in strong absorption of infrared light. In one of the visually dark equatorial patches, the methane ice has shallower infrared absorptions indicative of a very different texture. An Earthly example of different textures of a frozen substance: a fluffy bank of clean snow is bright white, but compacted polar ice looks blue.

New Horizons' surface composition team has begun the intricate process of analyzing Ralph data to determine the detailed compositions of the distinct regions on Pluto. This is the first detailed image of Pluto from the Linear Etalon Imaging Spectral Array, part of the Ralph instrument on New Horizons. The observations were made at three wavelengths of infrared light, which are invisible to the human eye. In this picture, blue corresponds to light of wavelengths 1.62 to 1.70 micrometers, a channel covering a medium-strong absorption band of methane ice, green (1.97 to 2.05 micrometers) represents a channel where methane ice does not absorb light, and red (2.30 to 2.33 micrometers) is a channel where the light is very heavily absorbed by methane ice. The two areas outlined on Pluto show where Ralph observations obtained the spectral traces at the right. Note that the methane absorptions (notable dips) in the spectrum from the northern region are much deeper than the dips in the spectrum from the dark patch. The Ralph data were obtained by New Horizons on July 12, 2015.

(Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) The Icy Mountains of Pluto New close-up images of a region near Pluto's equator reveal a giant surprise: a range of youthful mountains rising as high as 11,000 feet (3,500 meters) above the surface of the icy body. The mountains likely formed no more than 100 million years ago -- mere youngsters relative to the 4.56-billion-year age of the solar system -- and may still be in the process of building. That suggests the close-up region, which covers less than one percent of Pluto's surface, may still be geologically active today. The youthful age estimate is based on the lack of craters in this scene. Like the rest of Pluto, this region would presumably have been pummeled by space debris for billions of years and would have once been heavily cratered -- unless recent activity had given the region a facelift, erasing those pockmarks. Unlike the icy moons of giant planets, Pluto cannot be heated by gravitational interactions with a much larger planetary body. Some other process must be generating the mountainous landscape.

The mountains are probably composed of Pluto's water-ice "bedrock." Although methane and nitrogen ice covers much of the surface of Pluto, these materials are not strong enough to build the mountains. Instead, a stiffer material, most likely water-ice, created the peaks. The close-up image was taken about 1.5 hours before New Horizons closest approach to Pluto, when the craft was 47,800 miles (770,000 kilometers) from the surface of the planet. The image easily resolves structures smaller than a mile across. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Portion of Pluto's Sputnik Planum (Sputnik Plain) This annotated view of a portion of Pluto's Sputnik Planum (Sputnik Plain), named for Earth's first artificial satellite, shows an array of enigmatic features. The surface appears to be divided into irregularly shaped segments that are ringed by narrow troughs, some of which contain darker materials. Features that appear to be groups of mounds and fields of small pits are also visible.

This image was acquired by the Long Range Reconnaissance Imager (LORRI) on July 14 from a distance of 48,000 miles (77,000 kilometers). Features as small as a half-mile (1 kilometer) across are visible. The blocky appearance of some features is due to compression of the image. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Pluto Solar Occultations This figure shows the locations of the sunset and sunrise solar occultations observed by the Alice instrument on the New Horizons spacecraft. The sunset occultation occurred just south of the "heart" region of Pluto, from a range of 30,120 miles (48,200 km), while the sunrise occurred just north of the "whale tail", from a range of 35,650 miles (57,000 km). (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Portrait of Pluto and Charon The latest two full-frame images of Pluto and Charon were collected separately by NASA's New Horizons during approach on July 13 and July 14, 2015.

The relative reflectivity, size, separation, and orientations of Pluto and Charon are approximated in this composite image, and they are shown in approximate true color. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Frozen Plains in the Heart of Pluto's 'Heart' In the center left of Pluto's vast heart-shaped feature -- informally named "Tombaugh Regio" -- lies a vast, craterless plain that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes. This frozen region is north of Pluto's icy mountains and has been informally named Sputnik Planum (Sputnik Plain), after Earth's first artificial satellite. The surface appears to be divided into irregularly-shaped segments that are ringed by narrow troughs. Features that appear to be groups of mounds and fields of small pits are also visible. This image was acquired by the Long Range Reconnaissance Imager (LORRI) on July 14 from a distance of 48,000 miles (77,000 kilometers). Features as small as one-half mile (1 kilometer) across are visible.

The blocky appearance of some features is due to compression of the image. (Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) A Mountain Range within Pluto's 'Heart' A newly discovered mountain range lies near the southwestern margin of Pluto's heart-shaped Tombaugh Regio (Tombaugh Region), situated between bright, icy plains and dark, heavily-cratered terrain. This image was acquired by NASA's New Horizons' Long Range Reconnaissance Imager (LORRI) on July 14, 2015, from a distance of 48,000 miles (77,000 kilometers) and sent back to Earth on July 20. Features as small as a half-mile (1 kilometer) across are visible. These frozen peaks are estimated to be one-half mile to one mile (1-1.5 kilometers) high, about the same height as the United States' Appalachian Mountains. The Norgay Montes (Norgay Mountains) discovered by New Horizons on July 15 more closely approximate the height of the taller Rocky Mountains The names of features on Pluto have all been given on an informal basis by the New Horizons team.

(Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute) Pluto & Charon This view of Pluto was taken by the Hubble Space Telescope. It shows a rare image of tiny Pluto with its moon Charon, which is slightly smaller than the planet. Because Pluto has not yet been visited by any spacecraft, it remains a mysterious planet. Due to its great distance from the sun, Pluto's surface is believed to reach temperatures as low as -240°C (-400°F). From Pluto's surface, the Sun appears as only a very bright star. (Courtesy NASA) The following images represent the three newly designated dwarf planets. The image of Pluto was rendered from a map created from a servies of Hubble images. Eris is an artistic image, and Ceres is a Hubble telescope image. (Copyright Calvin J. Hamilton) Hubble Telescope Image This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by the Hubble Space Telescope (HST). The image was taken on February 21, 1994, when the planet was 4.4 billion kilometers (2.7 billion miles) from the Earth.

The HST corrected optics show the two objects as clearly separate and sharp disks. This now allows astronomers to measure directly (to within about 1 percent) Pluto's diameter of 2,320 kilometers (1,440 miles) and Charon's diameter of 1,270 kilometers (790 miles). The HST observations show that Charon is bluer than Pluto. This means that the worlds have different surface composition and structure. A bright highlight on Pluto indicates that it might have a smoothly reflecting surface layer. A detailed analysis of the HST image also suggests that there is a bright area parallel to the equator of Pluto. However, subsequent observations are needed to confirm that this feature is real. The new HST image was taken when Charon was near its maximum elongation from Pluto (0.9 arcseconds). The two worlds are 19,640 kilometers (12,200 miles) apart. (Courtesy NASA/ESA/ESO) The Surface of Pluto The never-before-seen surface of the distant planet Pluto is resolved in these NASA Hubble Space Telescope pictures.

These images, which were made in blue light, show that Pluto is an unusually complex object, with more large-scale contrast than any planet, except Earth. Pluto probably shows even more contrast and perhaps sharper boundaries between light and dark areas than is shown here, but Hubble's resolution (just like early telescopic views of Mars) tends to blur edges and blend together small features sitting inside larger ones. The two smaller inset pictures at the top are actual images from Hubble. North is up. Each square pixel (picture element) is more than 100 miles across. At this resolution, Hubble discerns roughly 12 major "regions" where the surface is either bright or dark. The larger images (bottom) are from a global map constructed through computer image processing performed on the Hubble data. Opposite hemispheres of Pluto are seen in these two views. (Courtesy NASA/ESA/ESO) Pluto, Charon, and USA Comparison This image shows the approximate size of Pluto and Charon by overlaying them on an Advanced Very High Resolution Radiometer (AVHRR) image of the United States of America.

Pluto is about 2274 kilometers (1410 miles) in diameter and Charon 1172 kilometers (727 miles) in diameter. The image of Pluto is based upon Hubble observations taken of Pluto in June and July of 1994. The Charon image is based upon photometric measurements acquired by Marc Buie of Lowell Observatory. (Copyright 1998 by Calvin J. Hamilton) Map of the Surface of Pluto This is the first image-based surface map of the solar system's most remote planet, Pluto. The map, which covers 85% of the planet's surface, confirms that Pluto has a dark equatorial belt and bright polar caps, as inferred from ground-based light curves obtained during the mutual eclipses that occurred between Pluto and its satellite Charon in the late 1980s. The brightness variations in this map may be due to topographic features such as basins and fresh impact craters. However, most of the surface features are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles and chemical byproducts deposited out of Pluto's nitrogen-methane atmosphere.

Names may later be proposed for some of the larger regions. Image reconstruction techniques smooth out the coarse pixels in the four raw images to reveal major regions where the surface is either bright or dark. The black strip across the bottom corresponds to the region surrounding Pluto's south pole, which was pointed away from Earth when the observations were made, and could not be imaged. (Courtesy NASA/ESA/ESO) Nordic Optical Telescope This image of Pluto was taken with the 2.6 meter Nordic Optical Telescope , located at La Palma, Canary Islands. It is a good example of the best imagery that can be obtained from Earth-based telescopes. (© Copyright Nordic Optical Telescope Scientific Association -- NOTSA) New Horizons: Pluto/Charon Mission Artist's concept of the New Horizons spacecraft during its planned encounter with Pluto and its moon, Charon. The craft's miniature cameras, radio science experiment, ultraviolet and infrared spectrometers and space plasma experiments would characterize the global geology and geomorphology of Pluto and Charon, map their surface compositions and temperatures, and examine Pluto's atmosphere in detail.

The spacecraft's most prominent design feature is a nearly 7-foot (2.1-meter) dish antenna, through which it would communicate with Earth from as far as 4.7 billion miles (7.5 billion kilometers) away. (Courtesy NASA/JHUAPL/SwRI) Why is Pluto no longer a planet? - BBC News BBC News Why is Pluto no longer a planet? By Paul Rincon Science editor, BBC News website 13 July 2015 Close share panel Image copyright IAU Image caption In 2006, Pluto was voted out of the planetary club by members of the International Astronomical Union Nasa's New Horizons mission made a close pass of Pluto this week. For more than 70 years, Pluto was one of nine planets recognised in our Solar System. But in 2006, it was relegated to the status of dwarf planet by the International Astronomical Union (IAU). So why was Pluto demoted? Where did the controversy start? Pluto was discovered in 1930 by US astronomer Clyde Tombaugh, who was using the Lowell Observatory in Arizona. Textbooks were swiftly updated to list this ninth member in the club.

But over subsequent decades, astronomers began to wonder whether Pluto might simply be the first of a population of small, icy bodies beyond the orbit of Neptune. This region would become known as the Kuiper Belt, but it took until 1992 for the first "resident" to be discovered. The candidate Kuiper Belt Object (KBO) 1992 QBI was detected by David Jewitt and colleagues using the University of Hawaii's 2.24m telescope at Mauna Kea. How did this change things? Confirmation of the first KBO invigorated the existing debate. And in 2000, the Hayden Planetarium in New York became a focus for controversy when it unveiled an exhibit featuring only eight planets. The planetarium's director Neil deGrasse Tyson would later become a vocal figure in public discussions of Pluto's status. But it was discoveries of Kuiper Belt Objects with masses roughly comparable to Pluto, such as Quaoar (announced in 2002), Sedna (2003) and Eris (2005), that pushed the issue to a tipping point.

Eris, in particular, appeared to be larger than Pluto - giving rise to its informal designation as the Solar System's "tenth planet". Image copyright NASA Image caption The discovery of other icy objects similar in size to Pluto forced a re-think by the IAU Prof Mike Brown of the California Institute of Technology (Caltech), who led the team that found Eris, would later style himself as the "man who killed Pluto" , while deGrasse Tyson would later jokingly quip that he had "driven the getaway car" . The finds spurred the International Astronomical Union to set up a committee tasked with defining just what constituted a planet, with the aim of putting a final draft proposal before members at the IAU's 2006 General Assembly in Prague. Under a radical early plan, the number of planets would have increased from nine to 12 , seeing Pluto and its moon Charon recognised as a twin planet, and Ceres and Eris granted entry to the exclusive club. But the idea met with opposition. Media captionDr Marek Kukula: "Reclassification changes the way we think about our corner of the galaxy" What happened next?

The discussions in Prague during August 2006 were intense, but a new version of a planetary definition gradually took shape. On 24 August, the last day of the assembly, members voted to adopt a new resolution outlining criteria for naming a planet: Image copyright IAU Pluto met the first two of the these criteria, but the last one proved pivotal. "Clearing the neighbourhood" means that the planet has either "vacuumed up" or ejected other large objects in its vicinity of space. In other words, it has achieved gravitational dominance. Because Pluto shares its orbital neighbourhood with other icy Kuiper Belt Objects, the resolution effectively stripped the distant world of a planetary designation it had held for some 76 years. It was immediately relegated it to the distinct category of "dwarf planet", alongside the biggest body in the asteroid belt, Ceres, and other large Kuiper Belt Objects such as Eris, Quaoar and Sedna.

Commenting at the time, the IAU's president of planetary systems science Prof Iwan Williams said: "By the end of the decade, we would have had 100 planets, and I think people would have said 'my goodness, what a mess they made back in 2006 '." Image copyright AFP Image caption Astronomer Jocelyn Bell-Burnell holds a toy of Disney's Pluto the dog during a vote on the planet's demotion in 2006 Was that the end of the matter? In a word, no. Some experts immediately questioned the part of the definition about a planet clearing its orbital neighbourhood. This is because Earth shares its cosmic turf with more than 12,000 near-Earth asteroids. Thus, some have argued that Earth, Jupiter and other planets also fail to meet the IAU's 2006 definition. Speaking just after the vote, Prof Alan Stern, chief scientist for the New Horizons mission, called the outcome "an awful decision" and described the new definition as "internally inconsistent".

Prof Owen Gingerich of Harvard, who chaired the planet definition committee, revealed that only 10% of the 2,700 scientists who had attended the 10-day meeting were present at the Pluto vote. The low turn-out has been blamed on timing; the vote was held on the last day of the General Assembly when many participants had left or were preparing to fly out from Prague. The debate has rumbled on ever since, on television, in the pages of books and in public talks. Most recently, Alan Stern challenged Neil deGrasse Tyson to a debate on the matter in 2014. But the latter expert turned down the offer, stating: "I don't have opinions that I require other people to have." The flyby of Pluto is unlikely to provide any information relevant to a change in Pluto's status. But it will bring into clear focus once more what is, and what isn't, meant by the term "planet". Five Years Later Artist's impression of Pluto and Charon as seen from one of Pluto's other moons. Credit: David Aguilar/Center for Astrophysics Five years ago today, the solar system lost a planet. On Aug.

24, 2006, Pluto — which had been known as the ninth planet since its 1930 discovery — was demoted to the newly created category of "dwarf planet." The decision was controversial, rankling some scientists who disagreed with the reasoning behind it. It also upset and confused many laypeople, who had regarded the nine planets as permanent fixtures in the sky — key touchstones for their understanding of the cosmos, and their place in it. But Pluto's reclassification shows that our knowledge of the world around us is always changing, that scientific truths aren't handed down from on high. And that reminder may be the greatest legacy of the longstanding debate about Pluto's planet status . "This debate shows people, especially kids, that science is always evolving, and it's exciting," said planetary scientist Scott Sheppard of the Carnegie Institution of Washington, who hunts for faraway dwarf planets. "And you should get involved in science, because there's a lot more to learn out there." [ Photos of Pluto and Its Moons ] Pluto is not alone When Pluto was discovered in 1930, it was viewed as an oddball, and that perception stuck for the next six decades.

Pluto, after all, is much smaller than the eight "traditional" planets, and its highly elliptical orbit is considerably out of whack with theirs. Pluto is also much farther away, zooming around the sun at an average distance of 3.65 billion miles (5.87 billion kilometers). [ Infographic: Pluto, A Dwarf Planet Oddity ] But in the 1990s, astronomers started realizing that Pluto isn't such an oddball after all. They began finding many other large objects in Pluto's domain, the ring of icy bodies beyond Neptune known as the Kuiper Belt . The new discoveries prompted some scientists to re-examine their basic understanding of the solar system's structure, including the way they'd classified its large objects. Just what should we call these icy, faraway bodies? If Pluto is a planet, then should they all be planets, too? This rethink really got a jump-start in 2005, when Caltech astronomer Mike Brown announced the discovery of Eris, a Kuiper Belt object that seemed to be even larger than Pluto. And shortly thereafter, the International Astronomical Union (IAU) stepped in.

Artist's rendering of Eris, announced in July 2005 by Mike Brown of Caltech. It is more massive than Pluto. The sun is in the background. Credit: NASA/JPL/Caltech Pluto's demotion In 2006, the IAU came up with the following official definition of "planet": A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies. Since Pluto shares the Kuiper Belt with lots of other big objects, it failed to meet the third criterion and didn't make the cut. Instead, the IAU rebranded Pluto and Eris as "dwarf planets." [ Meet the Solar System's Dwarf Planets ] Some planetary scientists generally agreed with the new classification, saying that Pluto, Eris and other big Kuiper Belt bodies don't belong in the same category as the eight "original" planets. The faraway icy objects, they say, are just too different — in their sizes, compositions, orbits and evolutionary histories. "It's classification that matters.

Classification is the first step toward understanding," Brown told SPACE.com. "If you mess up the most basic classification of the whole solar system by not noticing the difference between the eight large planets and everything else — if you miss that, you've kind of missed the solar system." Further, the word "planet" is so fundamental, and so central to any attempt to teach the public about the solar system, that defining it properly is crucial, Brown added. "If you said that there are eight planets and then a kajillion dwarf planets, that's a pretty good description of what the solar system's actually like," Brown said. "If you said, 'Wow, we've got this planetary system and there are 20,000 planets. Oh yeah, and eight of them are significantly bigger than all the rest of them' — it's not a very useful classification." Dissent and disagreement Other scientists, however, aren't at all happy with the IAU's planet definition, calling it flawed and unscientific. "I think the IAU really embarrassed themselves with this," said Alan Stern of the Southwest Research Institute in Boulder, Colo. Stern leads NASA's New Horizons mission , which is sending a spacecraft to study Pluto up close.

"They created a problem for themselves and for astronomy. It [the definition] created an unworkable algorithm for deciding what's a planet and what's not." [ Cosmic Definitions: What Is a Planet? ] Stern particularly objects to the "clearing your neighborhood" criterion. "In no other branch of science am I familiar with something that absurd," Stern told SPACE.com. "A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to." Further, Stern said, the criterion sets different standards for planethood at different distances from the sun. That's because the farther away a planet is from the sun, the bigger it needs to be to clear its zone. If Earth circled the sun in Pluto's orbit , for example, it wouldn't qualify for planethood in the IAU's eyes. "I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth," Stern said.

Stern doesn't have a problem with the term "dwarf planet." He just thinks dwarfs should be deemed "true" planets like the terrestrial planets and gas giants. Excluding the dwarfs from the official list amounts to an effort to keep the number of planets down to a manageable size, Stern added. "That's not a very scientific way of going about it, since we have countless numbers of stars, galaxies, asteroids and everything else," Stern said. Images of the dwarf planet Pluto taken by NASA's Hubble Space Telescope. Credit: NASA, ESA, and M. Buie (Southwest Research Institute) A better understanding of the solar system While scientists such as Brown and Stern may differ on the question of Pluto's planethood, they agree that the last two decades have brought about a sea change in our understanding of the solar system. For example, we now know that the solar system's frigid outer reaches are packed full of big, interesting objects. The first big Kuiper Belt body other than Pluto was discovered just in 1992, but Brown said up to 2,000 or so dwarf planets might actually exist out there. And many more big objects may lurk even farther away.

Right now, astronomers know of one mysterious probable dwarf planet, called Sedna, from the region beyond the Kuiper Belt . But that distant zone — which is very difficult to survey with today's instruments — may harbor up to 20,000 dwarf planets, Brown said. So Pluto is now known to be just one of many big, icy and quite diverse bodies orbiting the sun from far away. "We have gone from nearly complete ignorance about these sorts of bodies to an incredible richness, and trying to understand how they function as little worlds of their own," Brown said. Stern voiced similar sentiments, stressing that recent finds are showing planets — dwarf and otherwise, in our solar system and beyond — are fantastically diverse. "That's what we're finding in planetary science, as a result of not just the Kuiper Belt but also discoveries about extrasolar planets," Stern said. "There are planets that have densities like balsa wood, and planets that are darker than coal, and hot Jupiters and super-Earths .

I suspect we're just scratching the surface of the diversity — both in the Kuiper Belt and around other stars." The debate over Pluto's planethood is a result of this vast increase in knowledge, this revolution in how we view our cosmic neighborhood. And it serves as a reminder that science is not an ironclad set of facts but rather a process, a way to learn as much as possible about ourselves and our surroundings. "It shows that we're constantly expanding our knowledge of the solar system and our place in the universe," Sheppard told SPACE.com. "Science is always evolving, and we're always discovering new things." You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall . Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook . Editor's Recommendations

Question: In which year was the dwarf planet Pluto discovered?
Answer: 1930